.22 in a PCP (or multi-pump) for general shooting, pesting and small game hunting (rabbits and squirrels);
.177 for springers and SSPs for 10 meter target, casual target and plinking;
.25 for large pests (groundhogs, raccoons, porcupines and such).
If I had to choose one... considering BC, cost of pellets, selection of pellets, shot count, effective range (up to 40ish yards) and (reasonable) stability in breezy conditions, it would be a .22 caliber PCP. ...Love my Maximus, love my HW100FSB.
Surprised at how many mention the .20 caliber. I always considered it to be Beemans "proprietary caliber" pushed onto the airgun scene for marketing (not technical) reasons. Limited guns; limited pellet selection & availability (when they were introduced) and higher costs. Seen the .20 as a compromise useful in places where there is a FPE limit (like the 12 fpe in Britain) where it would have a bit more umph that the .177 and a bit better trajectory than the .22. I would be interested to hear the technical benefits of the .20 over the more traditional .177 and .22 calibers.